As shown in FIG. 4, a unidirectional condenser microphone unit has a cylindrically formed unit case 10 made of, for example, a brass alloy. On the front surface, which is directed to the sound source side, of the unit case 10, a front acoustic terminal 11 is provided, and on the side surface (circumferential surface) side thereof, a rear acoustic terminal 12 is provided to provide unidirectionality.
The front acoustic terminal 11 and the rear acoustic terminal 12 are openings for introducing sound waves into the unit case 10, and usually adopt round holes or slit holes.
In the unit case 10, an acousto-electric converter 20 is housed. This acousto-electric converter 20 is of an electrostatic type. Although not shown, in the acousto-electric converter 20, a diaphragm stretchedly provided on a diaphragm ring and a backplate supported on an insulating seat are disposed opposedly via an electrical insulating spacer ring.
The acousto-electric converter 20 is disposed between the front acoustic terminal 11 and the rear acoustic terminal 12 in the unit case 10, and an electrode terminal rod 21 connected electrically to the backplate is pulled out of the rear of the acousto-electric converter 20.
Since the electrostatic acousto-electric converter 20 has a very high impedance, the condenser microphone has an impedance converter 30 for converting sound signals to ones having a low impedance and sending out the converted sound signals. As the impedance converter 30, a field effect transistor (FET) is usually employed. Therefore, in the description below, the impedance converter is sometimes referred to as a FET 30.
The condenser microphone unit shown in FIG. 4 is used for a tie clip condenser microphone or a gooseneck condenser microphone. The FET 30 is housed in the unit case 10 in a state of being mounted on a circuit board 31, and the gate electrode thereof is connected electrically to the electrode terminal rod 21 via, for example, a plate spring 33.
To the circuit board 31, a microphone cord (not shown) is connected. The microphone cord is pulled out of the cord bush 13 side at the rear end of the unit case 10, and is connected to a power module section (not shown) including a sound signal output circuit and an output transformer.
Since the front acoustic terminal 11 and the rear acoustic terminal 12 are openings, a shielding member 40 having air permeability is provided on the front acoustic terminal 11 and the rear acoustic terminal 12 to prevent foreign matters from intruding into the unit case 10 and to prevent noise caused by the external electric field from being generated (for example, refer to Japanese Patent Application Publication No. S55-105492). In FIG. 4, the illustration of the shielding member provided on the front acoustic terminal 11 side is omitted.
The noise caused by the external electric field includes hum noise caused by a commercial power source at low frequencies and noise caused by broadcasting electromagnetic waves at relatively high frequencies. In recent years, noise caused by electromagnetic waves of very high frequencies radiated from cellular phones has posed a serious problem.
Usually, as the shielding member 40 of the rear acoustic terminal 12, a metal mesh 41 such as a stainless steel mesh (wire diameter: 0.1 mm, #100 mesh, material: SUS304) has been used. The metal mesh 41 is schematically shown in FIG. 5.
The metal mesh 41 is a plain-woven mesh body, so that the metal mesh 41 is electrically connected by contact points between longitudinal wires and transverse wires. For this reason, the electrical connection states of the contact points are not always fixed. Therefore, the shield performance of the metal mesh 41 is also nonuniform throughout the entire surface thereof. For example, the electrical resistance values at the contact points vary depending on whether the weaving is strong or weak.
As shown in FIG. 5, the metal mesh 41, which is formed by cutting a metal mesh into a band shape, is put into the unit case 10 by being rounded, and is mounted on the inner surface side of the rear acoustic terminal 12 by the elastic restoring force thereof. The metal mesh 41 mounted in this manner has a problem described below.
In the case where the length of the metal mesh 41 is shorter than the inner periphery length of the unit case 10, as shown in FIGS. 6A and 6B, a gap G1 is formed between both end parts of the metal mesh 41, and therefore the shield is incomplete in the part of this gap G1.
In contrast, in the case where the length of the metal mesh 41 is longer than the inner periphery length of the unit case 10, as shown in FIGS. 7A and 7B, both the end parts of the metal mesh 41 overlap with each other, and one end part thereof floats from the inner surface of the unit case 10 and a gap G2 is formed. Therefore, the shield is incomplete in the part of this gap G2.
If the metal mesh 41 is cut so as to fit to the inner periphery length of the unit case 10, the gaps G1 and G2 are not formed. However, this process requires a precise cutting machine or skilled work.
In addition, in the case where the restoring force of the metal mesh 41 is weak, and the contact pressure on the unit case 10 is low as well, the shield is incomplete.
Concerning the contact pressure, Japanese Patent Application Publication No. 2008-166909 proposes a technique in which a coil spring for pressing the metal mesh 41 against the inner surface of the unit case 10 is put in the unit case 10. According to this technique, the shield performance of the rear acoustic terminal 12 can be enhanced. However, this technique is unpreferable in terms of cost because of the need for the coil spring.
Accordingly, an object of the present invention is to provide a unidirectional condenser microphone unit in which the shield performance of the rear acoustic terminal is assured by simple assembling work without an increase in cost.